Cyclohexylbenzene derivatives are known as liquid crystal compounds (refer to Non-Patent Document 1). For example, a process for producing 4-(trans-4-heptylcyclohexyl)benzoic acid is disclosed (refer to Patent Document 1) and 4′-(4-propylhexyl)biphenyl-4-yl-acetate is disclosed (refer to Patent Document 2). Further, 4-(trans-4-pentylcyclohexyl)benzaldehyde (refer to Non-Patent Document 2), 4-(trans-4-proylcyclohexyl)benzaldehyde (refer to Patent Document 3), etc., are used. On the other hand, as a photopolymerization initiator, 1,2-bis(4-(4-propylcyclohexyl)phenyl)ethane-1,2-dione is disclosed (refer to Patent Document 4). Cyclohexylbenzene derivatives are also used in pharmaceutical applications and are understood to have a possibility to be used as functional materials in a variety of fields (refer to Patent Document 5).
Thus, cyclohexylbenzene derivatives are developed mainly for use as liquid crystal compounds and, additionally, have a possibility to be applied and developed in various fields such as those of pharmaceuticals and functional materials. In this circumstance, development of novel compounds is positively attempted.
In such technologies, a process for producing 4-(trans-4-pentylcyclohexyl)benzaldehyde by oxidation of 4-(trans-4-pentylcyclohexyl)benzyl alcohol with chromic acid is known (refer to Non-Patent Document 2). The known process is, however, not preferable as an industrial method, because 4-(trans-4-pentylcyclohexyl)benzyl alcohol as the raw material is not easily available and because chromic acid which is strongly toxic must be used. A process for producing 4-(trans-4-propylcyclohexyl)benzaldehyde by reducing 4-(trans-4-propylcyclohexyl)benzonitrile with NaAlH2(OC2H4OCH3)2 is also known (refer to Patent Document 3). This process is also problematic as an industrial method, because 4-(trans-4-propylcyclohexyl)benzonitrile as the raw material is not easily available, because a large amount of the expensive NaAlH2(OC2H4OCH3)2 must be used as a reducing agent, and because wastes such as Al(OH)3 are by-produced in a large amount.
A 4-(4-alkylcyclohexyl)benzaldehyde may also be produced from a (4-alkylcyclohexyl)benzene as follows. A (4-alkylcyclohexyl)benzene is first reacted with bromine in the presence of a reduced iron catalyst to obtain a 4-(4-alkylcyclohexyl)bromobenzene. Then, an ether solution of the refined 4-(4-alkylcyclohexyl)bromobenzene is added dropwise to an ether solution containing metallic magnesium. The resulting mixture is reacted with ethyl orthoformate to obtain a 4-(4-alkylcyclohexyl)benzaldehyde diethylacetal, which is finally hydrolyzed with an aqueous hydrochloric acid solution to obtain the 4-(4-alkylcyclohexyl)-benzaldehyde as aimed.
In addition to the desired trans isomer, however, the thus obtained aldehyde product tends to contain the cis isomer in an amount corresponding to that in the raw material. Because the boiling points of these isomers are very close to each other, it is impossible to separate and purify them by distillation. It may be considered that the trans-isomer is isolated by a crystallization method. Such a method is, however, not efficient and incurs a high cost. Thus, there is a demand for an effective method for selectively producing the trans isomer by controlling the reaction conditions.
In general, a cyclohexylbenzene derivative is produced as a mixture of the cis and trans isomers thereof. Therefore, it is necessary to adopt a method for separating the trans isomer from the mixture. As a method for separating the trans isomer from the isomeric mixture, a crystallization method is generally used as mentioned above. Such a method is, however, not efficient and incurs a high cost.
A method for efficiently producing the trans isomer by isomerization of the cis isomer into the trans isomer is also investigated. For example, Patent Document 6 and Patent Document 7 disclose a method of obtaining the trans isomer by isomerizing the cis isomer into the trans isomer using potassium t-butoxide. This method, however, must use a large amount of potassium t-butoxide and, therefore, has a problem that a treatment of an alkali waste liquor remaining after the reaction causes a great work load.
There is also disclosed a method in which a cis isomer is isomerized into a trans isomer in the presence of a Lewis acid, such as aluminum chloride, and an alkyl halide compound (refer to Patent Document 8). This method, however, poses a problem of treatment of aluminum chloride after the reaction and, additionally, must use a solvent such as methylene chloride which has high environment load.
Further, a method for isomerizing a cis isomer into a trans isomer using a hetero-polyacid is disclosed (refer to Patent Document 9). This method, however, requires a long reaction time and does not necessarily provide a high reaction efficiency.
Further, a method for isomerizing a cis isomer into a trans isomer using trifluoromethanesulfonic acid is disclosed (refer to Patent Document 10). This method is, however, not economical because a large amount of expensive trifluoromethanesulfonic acid must be used and, additionally, has a problem that methylene chloride which has high environment load must be used as an extraction solvent.
Non-Patent Document 1: Quarterly Chemical Review 22, “Chemistry of Liquid Crystals”, Yasuyuki GOTO, published by Academy Publication Center, Apr. 25, 1994, p. 40-59
Non-Patent Document 2: HELVETICA CHIMICA ACTA, Vol. 68 (1985), p. 1444-1452
Patent Document 1: Japanese Patent Application Laid-Open No. S56-077231
Patent Document 2: Japanese Patent Application Laid-Open No. S62-067049
Patent Document 3: Japanese Patent Application Laid-Open No. H03-141274
Patent Document 4: Japanese Patent Application Laid-Open No. H11-171816
Patent Document 5: Japanese Patent Application Laid-Open No. H06-293741
Patent Document 6: Japanese Patent Application Laid-Open No. H07-278548
Patent Document 7: Japanese Patent Application Laid-Open No. H09-278687
Patent Document 8: Japanese Patent Application Laid-Open No. H09-100286
Patent Document 9: Japanese Patent Application Laid-Open No. H07-41432
Patent Document 10: Japanese Patent Application Laid-Open (KOKAI) No. 2004-256490